1. Field of the Invention
The present invention relates to a light emitting system, more particularly to a light emitting system with light emitting power stabilization.
2. Description of the Related Art
The forward voltage of a light emitting diode (LED) is influenced by the ambient temperature. FIG. 1 shows a plot of forward voltage vs. ambient temperature obtained for each of a blue LED, a green LED, and a red LED that are driven by a constant driving current of 20 mA. It is evident that a rise in the ambient temperature will cause the forward voltage to fall, such that the light emitting power, or a product of the forward voltage and the operating current, is in a negative relation to the ambient temperature. Hence, application of an LED without implementation of light emitting power control may result in instability in the light emitting power.
Referring to FIG. 2, Taiwanese Patent Application No. 92107029 discloses a conventional light emitting power control circuit 1 for controlling a light emitting power of an LED 15 (e.g., a laser light emitting diode) in an optical pick-up of an optical drive device. The conventional light emitting power control circuit 1 includes a detection module 10, a signal source 11, an integration module 12, and a driving module 13.
The detection module 10 is operable to receive light emitted from the LED 15 and to detect the light emitting power of the LED 15 so as to generate a detection voltage (V3) having a magnitude that is in a positive relation to the light emitting power detected by the detection module 10. The light emitting power is defined by the equation of P=VF×I, where P, VF, and I are the light emitting power, a forward voltage, and an operating current of the LED 15, respectively.
The detection module 10 includes a light detector 101 and a front-end amplifier 102. Since a description of the operations of these components may be found in the specification of the aforesaid Taiwanese Application, these components will not be described hereinafter for the sake of brevity.
The signal source 11 is operable to generate a reference voltage (V1) that has a magnitude greater than that of the detection voltage (V3) and dynamically configurable according to a target light emitting power.
The integration module 12 is connected electrically to the signal source 11 and the detection module 10 for respectively receiving the reference voltage (V1) and the detection voltage (V3) therefrom, and is operable to output an integration voltage (V2) based on an integration of a difference between the reference voltage (V1) and the detection voltage (V3). When the detection voltage (V3) is reduced as a result of a reduction in the light emitting power, the difference between the reference voltage (V1) and the detection voltage (V3) is increased, causing the integration voltage (V2) to increase. On the other hand, when the detection voltage (V3) is increased as a result of an increase in the light emitting power, the difference between the reference voltage (V1) and the detection voltage (V3) is decreased, causing the integration voltage (V2) to decrease.
The driving module 13 is connected electrically to the integration module 12 for receiving the integration voltage (V2) therefrom, and is connected electrically to the LED 15 for providing to the LED 15 the operating current having a magnitude that is in a positive relation to the integration voltage (V2) received by the driving module 13. The driving module 13 includes an amplifier 131 having an adjustable gain, and a driving unit 132 electrically connected electrically to the amplifier 131. Since a description of the operations of these components may be found in the specification of the aforesaid Taiwanese Application, these components will not be described hereinafter for the sake of brevity.
When the forward voltage of the LED 15 is decreased as a result of an increase in the ambient temperature, the light emitting power is reduced, the detection voltage (V3) generated by the detection module 10 is decreased while the reference voltage (V1) remains unchanged, and the difference between the reference voltage (V1) and the detection voltage (V3) is thus increased such that the integration voltage (V2) and hence the operating current are, as a result, increased. This increase in the operating current serves to compensate for the reduction in the forward voltage, thereby achieving a light emitting power stabilization effect.
It can be understood from the above that the conventional light emitting power control circuit 1 stabilizes the light emitting power through adjusting the operating current according to variations in the detection voltage (V3), which correspond to variations in light detected by the light detector 101 of the detection module 10.
However, since the LED 15 suffers from poor directivity, factors such as distance between and positions of the light detector 101 and the LED 15, ambient light pollution, and sensitivity of the light detector 101 may cause errors in stabilization of the light emitting power, such that the conventional light emitting power control circuit 1 may not be able to effectively stabilize the light emitting power of the LED 15 in response to variations in the ambient temperature.